Jung and S¸ evli Advances in Difference Equations 2013, 2013:76 http://www.advancesindifferenceequations.com/content/2013/1/76
RESEARCH
Open Access
Power series method and approximate linear differential equations of second order Soon-Mo Jung1 and Hamdullah S¸ evli2* *
Correspondence:
[email protected] 2 Department of Mathematics, Faculty of Sciences and Arts, Istanbul Commerce University, Uskudar, Istanbul, 34672, Turkey Full list of author information is available at the end of the article
Abstract In this paper, we will establish a theory for the power series method that can be applied to various types of linear differential equations of second order to prove the Hyers-Ulam stability. MSC: Primary 34A05; 39B82; secondary 26D10; 34A40 Keywords: power series method; approximate linear differential equation; simple harmonic oscillator equation; Hyers-Ulam stability; approximation
1 Introduction Let X be a normed space over a scalar field K, and let I ⊂ R be an open interval, where K denotes either R or C. Assume that a , a , . . . , an : I → K and g : I → X are given continuous functions. If for every n times continuously differentiable function y : I → X satisfying the inequality an (x)y(n) (x) + an– (x)y(n–) (x) + · · · + a (x)y (x) + a (x)y(x) + g(x) ≤ ε for all x ∈ I and for a given ε > , there exists an n times continuously differentiable solution y : I → X of the differential equation an (x)y(n) (x) + an– (x)y(n–) (x) + · · · + a (x)y (x) + a (x)y(x) + g(x) = such that y(x) – y (x) ≤ K(ε) for any x ∈ I, where K(ε) is an expression of ε with limε→ K(ε) = , then we say that the above differential equation has the Hyers-Ulam stability. For more detailed definitions of the Hyers-Ulam stability, we refer the reader to [–]. Obłoza seems to be the first author who has investigated the Hyers-Ulam stability of linear differential equations (see [, ]). Thereafter, Alsina and Ger [] proved the HyersUlam stability of the differential equation y (x) = y(x). It was further proved by Takahasi et al. that the Hyers-Ulam stability holds for the Banach space valued differential equation y (x) = λy(x) (see [] and also [–]). Moreover, Miura et al. [] investigated the Hyers-Ulam stability of an nth-order linear differential equation. The first author also proved the Hyers-Ulam stability of various linear differential equations of first order (ref. [–]). Recently, the first author applied the power series method to studying the Hyers-Ulam stability of several types of linear differential equations of second order (see [–]). © 2013 Jung and S¸ evli; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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However, it was inconvenient that he had to alter and apply the power series method with respect to each differential equation in order to study the Hyers-Ulam stability. Thus, it is inevitable to develop a power series method that can be comprehensively applied to different types of differential equations. In Sections and of this paper, we establish a theory for the power series method that can be applied to various types of linear differential equations of second order to prove the Hyers-Ulam stability. Throughout this paper, we assume that the linear differential equation of second order of the form p(x)y (x) + q(x)y (x) + r(x)y(x) = ,
()
for which x = is an ordinary point, has the general solution yh : (–ρ , ρ ) → C, where ρ is a constant with < ρ ≤ ∞ and the coefficients p, q, r : (–ρ , ρ ) → C are analytic at and have power series expansions p(x) =
∞
pm xm ,
q(x) =
m=
∞
qm xm
and
m=
r(x) =
∞
rm xm
m=
for all x ∈ (–ρ , ρ ). Since x = is an ordinary point of (), we remark that p = .
2 Inhomogeneous differential equation In the following theorem, we solve the linear inhomogeneous differential equation of second order of the form p(x)y (x) + q(x)y (x) + r(x)y(x) =
∞
am xm
()
m=
under the assumption that x = is an ordinary point of the associated homogeneous linear differential equation (). m Theorem . Assume that the radius of convergence of power series ∞ m= am x is ρ > and that there exists a sequence {cm } satisfying the recurrence relation m (k + )(k + )ck+ pm–k + (k + )ck+ qm–k + ck rm–k = am
()
k=
m for any m ∈ N . Let ρ be the radius of convergence of power series ∞ m= cm x and let ρ = min{ρ , ρ , ρ }, where (–ρ , ρ ) is the domain of the general solution to (). Then every solution y : (–ρ , ρ ) → C of the linear inhomogeneous differential equation () can be expressed by
y(x) = yh (x) +
∞
cm xm
m=
for all x ∈ (–ρ , ρ ), where yh (x) is a solution of the linear homogeneous differential equation ().
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m Proof Since x = is an ordinary point, we can substitute ∞ m= cm x for y(x) in () and use the formal multiplication of power series and consider () to get p(x)y (x) + q(x)y (x) + r(x)y(x) =
∞ m
pm–k (k + )(k + )ck+ xm +
m= k=
+
∞ m
qm–k (k + )ck+ xm
m= k=
m ∞
rm–k ck xm
m= k=
=
∞ m
(k + )(k + )ck+ pm–k + (k + )ck+ qm–k + ck rm–k xm
m= k=
=
∞
am xm
m=
m for all x ∈ (–ρ , ρ ). That is, ∞ m= cm x is a particular solution of the linear inhomogeneous differential equation (), and hence every solution y : (–ρ , ρ ) → C of () can be expressed by
y(x) = yh (x) +
∞
cm xm ,
m=
where yh (x) is a solution of the linear homogeneous differential equation ().
For the most common case in applications, the coefficient functions p(x), q(x), and r(x) of the linear differential equation () are simple polynomials. In such a case, we have the following corollary. Corollary . Let p(x), q(x), and r(x) be polynomials of degree at most d ≥ . In particular, let d be the degree of p(x). Assume that the radius of convergence of power series ∞ m m= am x is ρ > and that there exists a sequence {cm } satisfying the recurrence formula m (k + )(k + )ck+ pm–k + (k + )ck+ qm–k + ck rm–k = am
()
k=m
for any m ∈ N , where m = max{, m – d}. If the sequence {cm } satisfies the following conditions: (i) limm→∞ cm– /mcm = , (ii) there exists a complex number L such that limm→∞ cm /cm– = L and pd + Lpd – + · · · + Ld – p + Ld p = , then every solution y : (–ρ , ρ ) → C of the linear inhomogeneous differential equation () can be expressed by
y(x) = yh (x) +
∞ m=
cm xm
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for all x ∈ (–ρ , ρ ), where ρ = min{ρ , ρ } and yh (x) is a solution of the linear homogeneous differential equation (). Proof Let m be any sufficiently large integer. Since pd+ = pd+ = · · · = , qd+ = qd+ = · · · = and rd+ = rd+ = · · · = , if we substitute m – d + k for k in (), then we have am =
d (m – d + k + )(m – d + k + )cm–d+k+ pd–k k=
+ (m – d + k + )cm–d+k+ qd–k + cm–d+k rd–k .
By (i) and (ii), we have lim sup |am |/m m→∞
d = lim sup (m – d + k + )(m – d + k + )cm–d+k+ m→∞ k= cm–d+k+ qd–k × pd–k + (m – d + k + ) cm–d+k+
/m cm–d+k cm–d+k+ rd–k + (m – d + k + )(m – d + k + ) cm–d+k+ cm–d+k+ d /m = lim sup (m – d + k + )(m – d + k + )cm–d+k+ pd–k m→∞ k=
d /m = lim sup (m – d + k + )(m – d + k + )cm–d+k+ pd–k m→∞ k=d–d
/m = lim sup(m – d + )(m – d + )cm–d + pd + Lpd – + · · · + Ld p m→∞
/m = lim sup pd + Lpd – + · · · + Ld p (m – d + )(m – d + ) m→∞
(m–d +)/m
× |cm–d + |/(m–d +) = lim sup |cm–d + |/(m–d +) , m→∞
which implies that the radius of convergence of the power series of this corollary immediately follows from Theorem ..
∞
m= cm x
m
is ρ . The rest
In many cases, it occurs that p(x) ≡ in (). For this case, we obtain the following corollary. Corollary . Let ρ be a distance between the origin and the closest one among singular m points of q(z), r(z), or ∞ m= am z in a complex variable z. If there exists a sequence {cm } satisfying the recurrence relation (m + )(m + )cm+ +
m (k + )ck+ qm–k + ck rm–k = am k=
()
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for any m ∈ N , then every solution y : (–ρ , ρ ) → C of the linear inhomogeneous differential equation y (x) + q(x)y (x) + r(x)y(x) =
∞
am xm
()
m=
can be expressed by
y(x) = yh (x) +
∞
cm xm
m=
for all x ∈ (–ρ , ρ ), where yh (x) is a solution of the linear homogeneous differential equation () with p(x) ≡ . Proof If we put p = and pi = for each i ∈ N, then the recurrence relation () reduces m to (). As we did in the proof of Theorem ., we can show that ∞ m= cm x is a particular solution of the linear inhomogeneous differential equation (). According to [, Theorem .] or [, Theorem ..], there is a particular solution y (x) of () in a form of power series in x whose radius of convergence is at least ρ . More m over, since ∞ m= cm x is a solution of (), it can be expressed as a sum of both y (x) and a solution of the homogeneous equation () with p(x) ≡ . Hence, the radius of convergence m of ∞ m= cm x is at least ρ . Now, every solution y : (–ρ , ρ ) → C of () can be expressed by
y(x) = yh (x) +
∞
cm xm ,
m=
where yh (x) is a solution of the linear differential equation () with p(x) ≡ .
3 Approximate differential equation In this section, let ρ > be a constant. We denote by C the set of all functions y : (–ρ , ρ ) → C with the following properties: m (a) y(x) is expressible by a power series ∞ m= bm x whose radius of convergence is at least ρ ; ∞ m m (b) There exists a constant K ≥ such that ∞ m= |am x | ≤ K| m= am x | for any x ∈ (–ρ , ρ ), where am =
m
(k + )(k + )bk+ pm–k + (k + )bk+ qm–k + bk rm–k
k=
for all m ∈ N and p = . Lemma . Given a sequence {am }, let {cm } be a sequence satisfying the recurrence formula () for all m ∈ N . If p = and n ≥ , then cn is a linear combination of a , a , . . . , an– , c , and c .
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Proof We apply induction on n. Since p = , if we set m = in (), then c =
r q a – c – c , p p p
i.e., c is a linear combination of a , c , and c . Assume now that n is an integer not less than and ci is a linear combination of a , . . . , ai– , c , c for all i ∈ {, , . . . , n}, namely, ci = αi a + αi a + · · · + αii– ai– + βi c + γi c , where αi , . . . , αii– , βi , γi are complex numbers. If we replace m in () with n – , then an– = c pn– + c qn– + c rn– + c pn– + c qn– + c rn– + ··· + n(n – )cn p + (n – )cn– q + cn– r + (n + )ncn+ p + ncn q + cn– r = (n + )np cn+ + n(n – )p + nq cn + · · · + (pn– + qn– + rn– )c + (qn– + rn– )c + rn– c , which implies cn+ =
n(n – )p + nq an– – cn – · · · (n + )np (n + )np –
qn– + rn– rn– pn– + qn– + rn– c – c – c (n + )np (n + )np (n + )np
n– = αn+ a + αn+ a + · · · + αn+ an– + βn+ c + γn+ c , n– where αn+ , . . . , αn+ , βn+ , γn+ are complex numbers. That is, cn+ is a linear combination of a , a , . . . , an– , c , c , which ends the proof.
In the following theorem, we investigate a kind of Hyers-Ulam stability of the linear differential equation (). In other words, we answer the question whether there exists an exact solution near every approximate solution of (). Since x = is an ordinary point of (), we remark that p = . Theorem . Let {cm } be a sequence of complex numbers satisfying the recurrence relation () for all m ∈ N , where (b) is referred for the value of am , and let ρ be the radius of m convergence of the power series ∞ m= cm x . Define ρ = min{ρ , ρ , ρ }, where (–ρ , ρ ) is the domain of the general solution to (). Assume that y : (–ρ , ρ ) → C is an arbitrary function belonging to C and satisfying the differential inequality p(x)y (x) + q(x)y (x) + r(x)y(x) ≤ ε
()
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for all x ∈ (–ρ , ρ ) and for some ε > . Let αn , αn , . . . , αnn– , βn , γn be the complex numbers satisfying cn = αn a + αn a + · · · + αnn– an– + βn c + γn c
()
for any integer n ≥ . If there exists a constant C > such that α a + α a + · · · + α n– an– ≤ C|an | n n n
()
for all integers n ≥ , then there exists a solution yh : (–ρ , ρ ) → C of the linear homogeneous differential equation () such that y(x) – yh (x) ≤ CKε for all x ∈ (–ρ , ρ ), where K is the constant determined in (b). Proof By the same argument presented in the proof of Theorem . with m stead of ∞ m= cm x , we have p(x)y (x) + q(x)y (x) + r(x)y(x) =
∞
∞
m= bm x
m
am xm
in-
()
m=
for all x ∈ (–ρ , ρ ). In view of (b), there exists a constant K ≥ such that ∞ ∞ m m am x ≤ K am x m=
()
m=
for all x ∈ (–ρ , ρ ). Moreover, by using (), (), and (), we get ∞ ∞ m am xm ≤ K am x ≤ Kε m=
m=
for any x ∈ (–ρ , ρ ). (That is, the radius of convergence of power series least ρ .) According to Theorem . and (), y(x) can be written as y(x) = yh (x) +
∞
cn xn
∞
m= am x
m
is at
()
n=
for all x ∈ (–ρ , ρ ), where yh (x) is a solution of the homogeneous differential equation (). In view of Lemma ., the cn can be expressed by a linear combination of the form () for each integer n ≥ . n Since ∞ n= cn x is a particular solution of (), if we set c = c = , then it follows from (), (), and () that ∞ n cn x ≤ CKε y(x) – yh (x) ≤ n=
for all x ∈ (–ρ , ρ ).
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Competing interests The authors declare that they have no competing interests. Authors’ contributions The authors declare that this paper is their original paper. All authors read and approved the final manuscript. Author details 1 Mathematics Section, College of Science and Technology, Hongik University, Sejong, 339-701, Republic of Korea. 2 Department of Mathematics, Faculty of Sciences and Arts, Istanbul Commerce University, Uskudar, Istanbul, 34672, Turkey. Acknowledgements Dedicated to Professor Hari M Srivastava. This research was completed with the support of The Scientific and Technological Research Council of Turkey while the first author was a visiting scholar at Istanbul Commerce University, Istanbul, Turkey. Received: 14 December 2012 Accepted: 4 March 2013 Published: 26 March 2013 References 1. Brillouet-Belluot, N, Brzde¸k, J, Cieplinski, K: On some recent developments in Ulam’s type stability. Abstr. Appl. Anal. 2012, Article ID 716936 (2012) 2. 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doi:10.1186/1687-1847-2013-76 Cite this article as: Jung and S¸ evli: Power series method and approximate linear differential equations of second order. Advances in Difference Equations 2013 2013:76.
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